Field of the Invention
Embodiments of the present invention relate to a drive train designed to be connected to an AC electric grid and including: an electric machine with a stator and a rotor, the stator being designed to be electrically connected to the alternating grid and having a stator frequency, a bidirectional system for converting one alternating electric current to another alternating electric current, designed to be connected between the alternating grid and the rotor of the electric machine, the bidirectional conversion system including: a bidirectional converter for converting from alternating current to direct current, designed to be connected to the alternating grid, a bidirectional inverter for converting from direct current to alternating current between the AC/DC converter and the rotor, the DC/AC inverter having controllable electronic switches designed to convert direct voltage into alternating voltage, the DC/AC inverter and the rotor being connected to each other at an intermediary point for each phase of the alternating voltage, and a device for controlling the switches of the DC/AC inverter according to a control law.
Embodiments of the present invention also relate to a setup for generating electric power to an alternating electric grid, the setup including a turbine and such a drive train connected to the turbine, on the one hand, and designed to be connected to the alternating grid, on the other. Embodiments of the present invention can be applied to a wind turbine or to a hydroelectric installation.
Description of Related Art
From the document “Grid connection of doubly fed induction generator wind turbines: a survey”, by Martinez de Alegria et al., a drive train of the aforementioned type is known. The drive train is connected to a three phase grid and includes a doubly fed electric machine, the stator of the machine being connected to the three phase grid and the rotor being fed by means of a three phase-three phase converter connected to the three phase grid. The converter comprises an AC/DC converter connected to the three phase grid and a DC/AC inverter connected between the AC/DC converter and the rotor of the machine. When a fault, such as a short circuit, is produced on the three phase grid, the drive train must follow certain rules imposed by the electric grid (from the English grid code), as for example the German electric grid, or yet the English electric grid, in order to allow the grid to override such fault (from the English Fault Ride Through—FRT).
Such a short circuit in the electric grid leads to a significant increase in the induction voltage to the rotor of the electric machine. The aforementioned document then foresees various solutions to avoid deterioration of the drive train, and, in particular, the converter, as a consequence of this increase in the induction voltage, while complying with the rules of the electric grid in question. Many solutions are foreseen, such as adding a protective device against excess voltage and/or excess current connected between the output terminals of the DC/AC inverter from the alternating side, or even adding thyristors connected in anti-parallel between the electric grid and the stator of the machine for each phase of the alternating current. One solution is to add the protective device against excess voltage and/or excess current in combination with a rotor flow check device by means of a special control for the converter connected between the rotor and the grid.
However, these various solutions require attaching to the doubly fed electric machine an additional active protective device, such as a protective device against excess voltage and/or excess current, or thyristors connected in anti-parallel between the electric grid and the stator of the machine; such a device is expensive.